Well I've identified a few categories of issues that may occur for you due to either installation/configuration errors or hosting issues. Here are a list that I've found:
1. Class ACP does not exist error:
This is what you commonly notice in error log if you get an ACP does not exist fatal error.

Quote:

Error: Class ACP either does not exist!' in .../classes/class_loader.php:26

As you know, the classes in Admincp folder are all prefixed by ACP, but the script cannot find the corresponding class. This is usually because you've specified the incorrect script path, since in ACP I use a url parser that maps your script path to a certain system variable. Try editing script path configuration in your inc/config.php and see if this works. Keep in mind that the url parser is case-sensitive, for instance Felkyo and felkyo are not the same. Also check whether you add extra slashes in scriptpath.

2. No permission error for whatever action after you have logged into ACP:
For users who get this error message(not a fatal error), do you happen to notice the 'www.' appearing in your browser url? If so, this may be a problem for those who happen to use subdomains in the format of subdomain.domain.com, as the 'www.' part aint working out for them. The script cannot tell that www.yourdomain.com and yourdomain.com are the same domain/site, which is the cause of this problem.

To fix this issue, go to classes/class_page.php and get rid of the 'www.' you see in that script file. It should work out for you then, you may want to get rid of the 'www.' in your class_pagination.php too since you may encounter a pagination error later on.

3. Miscellaneous Errors:
If your problem do not belong to the previous two categories, there may be some issues with your hosting service. Do you happen to receive PDO not found error? If so, tell your host to install PDO extension or move. Also you may want to ask your server provider whether they have mod_rewrite enabled. In most cases they should, but I do realize that in a few rare circumstances they are being stupid enough not to have mod_rewrite. For those hosts, stay away from them as their PHP/Apache installation is incomplete. This will be a problem for ALL applications you use, not just for Mysidia Adoptables. On the other hand, read error_log from the folder /admincp incase your host has disabled error reporting, since this is where you normally find what the cause of your problem is.